Crust fungus

Ustulina deusta, also known as the crust fungus or carbon cushion, is a serious wood-decaying pathogen belonging to the Hypoxylaceae family. It is best known for causing brittle white rot in a wide range of broad-leaved trees. In its modern taxonomic classification, it is frequently placed within the genus Kretzschmaria, though the name Ustulina deusta remains widely recognized in botanical and forestry studies.

The host range of this pathogen is extensive, including beech, oak, lime, maple, and various orchard trees like apple and pear. The fungus acts primarily as a parasite, entering the tree through wounds or root damage. Once established, it causes a distinctive degradation of the wood structure, making the trunks highly susceptible to structural failure and windthrow.

The life cycle of Ustulina deusta involves the formation of charcoal-like, crusty stromata at the base of affected trees or on decaying stumps. These structures release spores, which are disseminated by wind to colonize new hosts. The mycelium penetrates the bark and spreads internally, slowly breaking down cellulose and lignin, which are the main structural components of wood.

Conditions favorable for the spread of this fungus include warm, humid environments and sites with poor tree vigor or historical physical damage. Because the fungus acts from the inside out, the tree may appear healthy until the decay has significantly compromised its stability. This "hidden" nature makes early detection critical for both commercial orchards and urban forestry.

Management and control strategies focus on prevention and sanitation:

• immediate removal of infected trees to eliminate the source of spores;
• proper pruning techniques to minimize wound size and expedite healing;
• sealing pruning cuts and mechanical wounds with protective substances;
• avoiding soil compaction and root damage, which act as entry points for the pathogen.